Electrostatic printing



United States Patent 3,053,688 Patented Sept. 11, 1962 3,053,688ELECTROSTATIC PRINTING Harold G. Greig, Princeton, N.J., assignor toRadio Corporation of America, a corporation of Delaware No Drawing.Filed Apr. 13, 1959, Ser. No. 805,740 19 Claims. (Cl. 117-37) Thisinvention relates generally to electrostatic printing. Moreparticularly, it relates to improved materials and methods fordeveloping electrostatic images.

In the art of electrostatic printing, electrostatic images are producedon the surface of an insulating material. Such images comprise a patternof electrostatic charges on the surface. Visible images are commonlyproduced therefrom by cascading across the surface a dry mixture offinely-divided developer particles and substantially larger carrierparticles. When the developer particles are tiiboelectrically-charged inthe opposite polarity to the electrostatic charges, they deposit incharged areas to produce a visible image in substantial configurationwith the pattern of charges. When the developer particles have the samepolarity as the electrostatic charges a visible image is produced inreverse configuration with respect to the pattern of charges.

The foregoing method of developing electrostatic images is described inElectrofax Direct Electrophotographic Printing on Paper, by C. J. Youngand H. G. Greig, RCA Review, December 1954, vol. XV, No. 4. Alsodescribed in that publication are other methods of development such as:powder cloud, liquid mist and magnetic brush types.

The recording element may comprise almost any insulating surface but,preferably, the recording surface is also photoconductive to enable therecording of light images. Recording elements comprising photoconductiveselenium coated plates are described in US. Patent 2,297,691, issuedOctober 6, 1942 to C. F. Carlson. Recording elements comprisingphotoconductive coatings on paper are described in the Young and Greigpublication, op. cit.

Recently, a so-called liquid process for developing electrostatic imageshas been proposed in which the solid developer particles are suspendedin an insulating carrier liquid. Liquid development methods provide manydistinct advantages over the use of dry developer mixtures and othermethods of developing electrostatic images, for some applications.Basically, the liquid developer previously described consists offinely-divided developer particles dispersed in a hydrocarbon liquid.This developer can be flowed over a surface bearing an electrostaticimage, or the surface can be immersed in a tray of liquid developer. Itcan also be sprayed or rolled on to the surface. When appropriatedeveloper particles are dispersed in a properly selected liquid, theyacquire an electrophoretic or triboelectric charge enabling them to beattracted to an electrostatic charge pattern of appropriate polarity.Deposition of the developer particles on the charge image is an exampleof the phenomenon known as electrophoresis or cataphoresis. A liquiddeveloper process for charge images is described in greater detail by K.A. Metcalf and R. J. Wright in a paper entitled Xerography, published inthe Journal of the Oil and Colour Chemists Association, November 1956,volume 39, No. 11, London, England and in another paper entitled LiquidDevelopers for Xerography published in the Journal of ScientificInstruments, February 1955, vol. 32.

Although the above-mentioned liquid developer compositions are suitablefor many purposes, they do possess undesirable properties. Mosthydrocarbon liquids are solvents for developer particles which includeresins and waxes or organic pigments. When resinous particles aredispersed in such a liquid, they dissolve at least to some extent sothat they become tacky and tend to agglomerate. Thus, such dispersionsmust be freshly made a short time prior to use. If the dispersions standfor any extended period of time, the developer particles will ball up orcake. The tackiness of the developer particles caused by the hydrocarbonliquid can also make them adhere in unwanted image areas which they maycontact during development. Unless some fixative spray is applied to animage developed with such a dispersion, the tacky developer particleswill tend to smear during handling. To improve the quality of thedeveloped image and to accelerate drying of the surface on which theimage rests, it is frequently desirable to heat that surface to driveoff the liquid and diffuse the developed image into the base material.When hydrocarbon carrier liquids are employed, such a practice would beextremely dangerous in view of the fire hazard involved. Also, whetherheated or not, most hydrocarbon liquids have a very objectionable odorand the vapors thereof are toxic. Thus, the use of such liquids callsfor employing expensive auxiliary equipment such as exhaust hoods inorder to remove fumes. For the above reason it can be readily seen thatsuch liquids are unsuitable for many applications such as, for example,office copiers.

Accordingly, it is a general object of this invention to provideimproved liquid developer mixes for electro static printing.

It is a further object of this invention to provide an improved liquiddeveloper composition which may, without danger of fire, be heated on asurface to fuse thermoplastic particles contained in the liquid, to thesurface.

It is a further object of this invention to provide improved liquiddeveloper compositions which are relatively odorless and non-toxic.

The foregoing objects and other advantages are accomplished inaccordance with this invention which provides improved liquidcompositions for developing electrostatic images. The compositionscomprise dispersions of finelydivided developer particles in a liquidconsisting essentially of a dimethyl polysiloxane, the developerparticles being insoluble in the liquid. Also contemplated is thedevelopment of electrostatic images by applying thereto such a liquiddispersion wherein thermoplastic developer particles are employed.Subsequent to development, the developed image is heated to removeliquid on the surface on which it rests and to fuse thereon thedeveloped image.

Specific examples and additional advantages of the liquid developers andof the improved methods of developing electrostatic images in accordancewith this invention are included in the detailed description whichfollows:

Dimethyl Polysiloxanes An important feature of this invention is theprovision of a carrier liquid consisting essentially of at least onedimethyl polysiloxane. Such compounds have a structural formulas whereIt may vary from 0 to 2000 and even higher. The higher the value of n,the higher the viscosity of the liquid at a given temperature. At roomtemperature, viscosity may vary from as low as 0.65 centistoke to ashigh as 1,000,000 centistokes, but for the purpose of the presentinvention, it is preferred to use only those members of the family ormixtures thereof having a viscosity up to about 3 centistokes at roomtemperature.

It has now been found that these liquids are extremely useful forelectrostatic printing for the following reasons. The liquids are verypoor solvents for organic plastics. The members of this family ofcompounds have relatively high flash points. For example, a dimethylpolysiloxane having a viscosity of 2 centistokes, has a flash point of175 F. and one of 3 centistokes a flash point of 215 F. This compareswith toluene at 40 F., turpentine at 90 F. and high-flash naphtha at 112F. The dimethyl polysiloxanes are practically odorless and non-toxicmaking them feasible for use in ofi'ice copiers. The insulating organicliquids such as benzene, toluene, turpentine, petroleum fractions,carbon tetrachloride, cyclohexane, etc. which have previously beendescribed as being suitable for electrophoretic development inelectrophotography, are all deficient with respect to one or more of theproperties mentioned above. The dimethyl polysiloxanes are alsoextremely hydrophobic and have excellent dielectric properties. This isof particular advantage in the present invention since most organicliquids lose their insulating properties in atmospheres of high relativehumidity, whereupon they become too conductive for effective operation.

Developer Mixes To prepare a suitable developer composition,finelydivided particles of an electroscopic developer material, aredispersed in dimethyl polysiloxane in a proportion of 20 parts by weightof developer material to 80 parts by weight of a dimethyl polysiloxanehaving a viscosity of about 0.6 to about 3 centistokes. The ratio ofdeveloper material to liquid in this example is generally too high formost applications. However, this ratio is a convenient one for preparinga composition which is to be stored for an extended period or which isto be provided to the ultimate user. Prior to use, the mixture isdiluted with additional dimethyl polysiloxane to provide a concentrationof developer material in the composition of about 0.2% to about 6% byweight. An important property of dimethyl polysiloxane evidences itselfwhen developer compositions are stored for extended periods. Developerparticles comprising thermoplastic materials, examples of which areprovided hereinafter, may be dispersed in such liquids and storedpractically indefinitely without agglomerating.

A specific example of a suitable toner or developer material to bedispersed in the dimethyl polysiloxane comprises the following:

EXAMPLE I 200 parts by weight of Piccolastic Resin 4358A (an elasticthermoplastic resin composed of polymers of styrene, substituted styreneand its homologs of the Pennsylvania Industrial Chemical Corp, Clairton,Pennsylvania) 12 parts by weight Carbon Black 12 parts by weightNigrosine SSB-Color Index No.

8 parts by weight Iosol Black-Color Index Solvent Black 13 Thisdeveloper material is prepared by melting the resin and mixing in theother materials. When a uniform mix is obtained, it is cooled, ground toa fine powder and classified to obtain a desired particle size. Aconvenient particle size is one obtained by screening through a 200 meshwhich provides a maximum particle diameter of about 74 microns. Thisdeveloper material may be dispersed in liquid by any of the commonlyknown techniques.

EXAMPLE II A low-melting point (120 C.) developer material suitable fordispersion in a dimethyl polysiloxane may be comprised as follows:

60 parts by weight Piccolastic D 100 40 parts by weight Piccolastic C125 9 parts by weight Carbon Black These materials are mixed together inpowder form, then melted and mixed again to obtain a homogeneousdispersion. "The melt is then cooled, ground and classified to obtainthe desired particle size. It has been found that even with alow-melting toner of this character, which has a tendency to cake withstorage, a stable noncaking dispersion is obtained in a dimethylpolysiloxane having a viscosity of about 2 centistokes.

In the foregoing examples many organic resins and waxes may besubstituted for those described. Some of these are the following:

Acrawax C (a synthetic waxoctadecenamide) The Glycol Products Co.,Brooklyn, N.Y.--melting point between 133 and 140 C.

Carnauba Wax-melting point about C.

Polymekon Wax (a commercially modified microcrystalline wax of theWarwick Wax Co., N.Y.)melting point about 93 to 127 C.

Ultracera Amber Wax-a microcrystalline petroleum wax of the Bareco WaxCo., Barnsdall, Oklahomamelting point between about 108 and 112 C.

Be Square Wax White-a microcrystalline petroleum wax of the Bareco WaxCo., Barnsdall, Oklahomamelting point between about and 109 C.

Petronauba D Wax-a microcrystalline petroleum wax of the Bareco WaxCo.melting point about 103 C.

Piccolyte S-a thermoplastic hydrocarbon of the Pennsylvania IndustrialChemical Co., Clairton, Pa.-- melting point about 135 C.

Various coloring agents may be employed, singly or in combination, inthe foregoing compositions in place of the black pigments or dyesspecifically set forth in Examples 1 and II. Colored developer particleswill generally include from .2 to 12 parts by weight of a coloring agentfor each 100 parts by weight of developer particles. Suitable coloringagents include the following:

(1) Cyan Blue Toner GT (described in US. Patent 2,486,351 to R. H.Wiswall, Jr.)

(2) Benzidine Yellow (3) Brilliant Oil Blue BMA, Color Index No. 61555(4) Sudan III Red, Color Index No. 26100 (5) Oil Yellow 2G, Color IndexNo. 11020 (6) Hansa Yellow G, Color Index No. 11680 Coated ParticlesVarious thermoplastic developer materials which comprise coatedparticles may also be conveniently employed in accordance with thisinvention. It is preferred in such cases to incorporate in the particlesa core material made up of zinc oxide. One type of zinc oxide, whenfused onto a surface by means of the thermoplastic coating is incapableof retaining an electrostatic charge. Such a zinc oxide has a value ofsurface photoconductivity less than 10* ohm- /square/watt/cm'. whensubjected to light of a wavelength of about 3900 A. When a fused visibleimage is produced with this zinc oxide it cannot be overprinted insubsequent operations.

Another suitable type of Zinc oxide comprises one having a value ofsurface photoconductivity of at least 10- ohm-Vsquare/watt/cm. whensubjected to light of a wavelength of about 3900 A. These developermaterials are convenient for use in color processes wherein one color isoverprinted over another to provide for color mixing. When particles oftoo large diameter and which are insulating in character are depositedon an electro static image to produce a first color, such particles willprohibit over-printing thereon with another color. By providing aphotoconductive zinc oxide core coated with a low-melting thermoplasticcoating, developer particles are produced which, when fused to asurface, permit overprinting of a color with another and thereforeprovide for color mixing.

The process by which one type of coated zinc oxide particles provide foroverprinting and by which another type prohibits overprinting is unique.When particles of either type are fused to a surface, the coatingmaterial melts to form a continuous layer adhering to the surface. Afterfusing, at least the topmost particles of zinc oxide are left protrudingabove the layer. When photoconductive Zinc oxide particles are employed,an image surface is produced which can be charged, exposed andoverprinted as easily as an original photoconductive surface. Whennon-photoconductive zinc oxides are employed, developed image areas areincapable of retaining a charge and, hence, cannot be overprinted insubsequent procedures.

Examples of suitable materials include the following:

EXAMPLE III White Developer Powder 1 part by weight carnauba wax 2 partsby weight photoconductive zinc oxide The wax is melted and particles ofzinc oxide having a particle size from 0.25 to .5 micron mean diameterare added to the belt. Particle size and shape of the zinc oxidedetermine to some extent the ratio of the Zinc oxide to the coatingmaterial. Continuous stirring of the melt from to minutes is sufficientto disperse the zinc oxide in the wax when the batch weighs about 100grams. The mixture is allowed to cool, after which it is reduced to afine powder and classified as to particle size.

EXAMPLE IV Blue Developer Powder 20 parts by weight Acrawax C (asynthetic wax-octadecenamide, of the Glyco Products Co., Brooklyn, NewYork) 30 parts by weight photoconductive zinc oxide 0.3 part by weightcalcium stearate (pigment wetting agent) 1.5 parts by weight Cyan BlueToner GT This composition is prepared the same as in Example III exceptthat the calcium stearate is added to the melt before the Zinc oxide andthe coloring agent after the zinc oxide.

EXAMPLE V Red Developer Powder 36 parts by weight Acrawax C 5 parts byweight of a solid silicone resin (such as Dow Corning R-5071) 80 partsby weight photoconductive zinc oxide 4 parts by weight Sudan 3 Red,Color Index No. 26100 2 parts by weight Oil Yellow 2 G Color Index No.

Preparation same as in Example IV.

Pigments It is possible to provide developer compositions which consistof organic pigments dispersed in dimethyl polysiloxane liquid.Preferably the dispersions comprise up to about 20 parts by weight ofpigment the remainder being liquid. The term pigment as employed hereinand in the claims is intended to include coloring agents which aresometimes referred to as dyes but which nevertheless are insoluble inthe polysiloxane. When used as taught herein these so-called dyes haveall the properties and attributes of pigments. Suitable pogments forsuch purposes include the following:

(1) Cyan Blue Toner GT (described in US. Patent 2,486,351 to Richard H.Wiswall, Jr.) (2) Benzidine Yellow (Color Index No. 21090) (3) BrilliantOil Blue BMA-Color Index No. 61555 (4) Sudan 3 Red-Color Index No. 26100(5) 'Oil Yellow 2 GC.I. No. 11020 (6) Pyrazalone pigment. (Such as C.I.21080 C.I. Pigment Red 39) (7) Hansa Yellow GC.I. No. 11680 In many ofthe foregoing dispersions it is convenient to provide a surfactant(surface active agent) to enhance the electrical properties of aselected pigment. A surfactant solution may be prepared by dispersing 10grams of Nalcamine Gl4 in 20 grams of toluene and, while mixing, heatingthe dispersion to dissolve the Nalcamine G14 in the toluene. NalcamineG14 is a chemical of the type 1-(2-hydroxyethyl)-2-hydrogenatedtallow-2-imidazoline (National Aluminate Corp, Chicago, Illinois). TheNalcamine G14 solution is added to pigment dispersions before they areball milled in the dimethyl polysiloxane. Such a surfactant whenapplied, for example, to a red pyrazalone pigment substantially enhancesthe electropositive nature thereof.

Reversal Type Powders This invention also provides a developercomposition which is capable of producing reverse images. By this ismeant that when the composition is applied to an electrostatic imageconsisting of a pattern of negative electrostatic charges, the developermaterial will adhere in.

non-charged areas of the image rather than in the charged areas thereof.Such a developer composition may be prepared by dispersing a pigment ina binder material such as one which is predominantly comprised ofpolyvinyl chloride.

EXAMPLE VI 4 grams carbon black 30 grams dimethyl polysiloxane,viscosity about 2 centistokes The carbon black is dispersed in thepolysiloxane and the dispersion ball milled in a 2 ounce glass jar withsteel balls for about 40 hours. The reversal type developer compositionis then made up as follows:

3 grams carbon black dispersion in polysiloxane 5 grams of Vinylite VYNV(a copolymer, 96% vinyl chloride and 4% vinyl acetate) 30 grams dimethylpolysiloxane employing most of the pigments discussed heretofore.v Someof these are set forth below along with the proportions of theconstituents therein.

EXAMPLE VII Red Reversal Powder 11 grams Vinylite V YNV 2 grams redpyrazalone pigment 30 grams dimethyl polysiloxane, viscosity about 2centistokes This mixture is ball milled for about 32 hours and may bediluted with additional polysiloxane.

EXAMPLE VIII Yellow Reversal Powder 14 grams Vinylite V YNV 2 gramsHansa Yellow G 40 grams dimethyl polysiloxane, viscosity about 2centistokes Preparation the same as for Example VI I.

7 EXAMPLE IX Blue Reversal Powder 12 grams Vinylite VYNV 2 grams PatentBlue 30 grams dimethyl polysiloxane, viscosity about 2 centistokesPreparation the same as in Example VII.

Use of any of the abovedescribed developer compositions in electrostaticprinting processes as contemplated in this invention provides for newand substantially improved results. In accordance with this invention,the methods call for applying the developer composition to theelectrostatic image by such means as, for example, flowing across theimage, spraying, application with a roller or by immersing the image ina tray containing the liquid composition. When an electrostatic image isdeveloped in this manner, the improved results are immediately evidencedin that there is far less deposition of developer particles in unwantedareas of the image than was hitherto deemed possible. By the simple stepof heating the surface on which the developed image resides to atemperature above the melting point of the developer particles, excesscarrier liquid is driven from the surface and the developer particlesare fused thereto. By this method a durable image is formed which canwithstand repeated handlings without smearing and which when applied toa flexible surface will flex with that surface rather than to peel orchip therefrom. During the heating step another unusual property of thedimethyl polysiloxane manifests itself. Although it may have a flashpoint of only about 175 F. it can, without danger of fire, be heated toa temperature of 425 F. or even more.

What is claimed is:

1. A composition of matter comprising a dispersion of electroscopicparticles having a diameter not larger than about 74 microns in a liquidconsisting essentially of a dimethyl polysiloxane having a viscosity ofbetween about 0.6 and about 3 centistokes, said particles beingsubstantially insoluble in said liquid, the concentration of saidparticles in said composition being no greater than about 20% by weight.

2. The composition of claim 1 wherein the concentration of saidparticles in said composition is no greater than about 6% by weight.

3. A composition of matter comprising a dispersion of electroscopicparticles having a diameter not in excess of about 74 microns, saidparticles comprising a material selected from the class consisting ofnatural and synthetic waxes and resins, in a liquid consistingessentially of a dimethyl polysiloxane having a viscosity between about0.6 and about 3 centistokes, the concentration of said particles in saidcomposition being no greater than about 20% by weight.

4. The composition of claim 3 wherein the concentration of saidparticles in said composition is no greater than about 6% by weight.

5. The composition of claim 4 wherein said particles comprise athermoplastic material having a melting point less than about 200 C.

6. The composition of claim 5 wherein said thermoplastic material issubstantially colorless and has coloring matter incorporated therein.

7. A composition of matter comprising a dispersion of particles of zincoxide coated with an electroscopic material selected from the classconsisting of natural and synthetic waxes and resins having a meltingpoint of from about 90 C. to about 200 C., in a liquid consisting es- 7sentially of a dimethyl polysiloxane having a viscosity of about 0.6 toabout 3 centistokes, the concentration of said particles in saidcomposition being no greater than about 20% by weight.

8. The composition of claim 7 wherein the concentration of said coatedparticles in said composition is no greater than about 6% by weight.

9. The composition of claim 8 wherein said electroscopic material issubstantially colorless and has coloring matter incorporated therein.

10. A composition of matter comprising a dispersion in a liquid ofparticles of photoconductive zinc oxide having a diameter not largerthan about 74 microns, said particles having a coating thereon of athermoplastic electroscopic material having a melting point within arange of from about C. to about 200 C., said liquid consistingessentially of a dimethyl polysiloxane having a viscosity of from about0.6 to about 3 centistokes, said thermoplastic material beingsubstantially insoluble in said liquid, the concentration of saidparticles in said composition being no greater than about 20% by weight.

11. The composition of claim 10 wherein the concen tration of saidparticles in said composition is no greater than about 6% by weight.

12. The composition of claim 10 wherein said electroscopic material isselected from the class consisting of substantially colorless naturaland synthetic waxes and resins and has coloring matter included therein.

13. A composition of matter comprising a dispersion in a liquid ofcolored electroscopic particles having a diameter not larger than about74 microns, said particles comprising a pigmented resinous material themajor proportion of which is polyvinyl chloride, said liquid consistingessentially of a dimethyl polysiloxane having a viscosity of from about0.6 to 3 centistokes, the concentration of said particles in saidcomposition being no greater than about 20% by weight.

14. The composition of claim 13 wherein the concentration of saidparticles in said composition is no greater than about 6% by weight.

15. The composition of claim 13 wherein said resinous material comprisesat least 90% by weight of polyvinyl chloride.

16. The composition of claim 13 wherein said resinous material is acopolymer consisting essentially of about 96% by weight of vinylchloride and 4% by weight of vinyl acetate.

17. In a method of developing an electrostatic image on an insulatingsurface, the improvement comprising applying to said surface a developercomposition consisting essentially of finely-divided electroscopicthermo plastic particles dispersed as a phase in a carrier liquidconsisting essentially of a dimethyl polysiloxane having a viscosity ofbetween about 0.6 and 3 centistokes, depositing said thermoplasticparticles on said surface by electrostatic attraction thereto, andheating said surface to fuse said particles thereto and remove saiddimethyl polysiloxane therefrom.

18. In a method of developing an electrostatic image on an insulatingsurface, said image comprising a pattern of negative electrostaticcharges, the improvement comprising: applying to said surface adeveloper composition consisting essentially of pigmented thermoplasticdeveloper particles dispersed as a phase in a dimethyl polysiloxaneliquid having a viscosity of between about 0.6 and 3 centistokes;depositing said developer particles on said pattern of negativeelectrostatic charges; and, heating said surface to fuse said developerparticles thereto and to remove said dimethyl polysiloxane therefrom.

19. In a method of developing an electrostatic image on an insulatingsurface, said image including areas hearing negative electrostaticcharges, the improvement comprising: applying to said surface adeveloper composition comprising pigmented developer particles, themajor proportion of which is polyvinyl chloride, dispersed as a phase ina dimethyl polysiloxane liquid having a viscosity of about from 0.6 to 3centistokes; depositing said developer particles in areas on saidsurface other than said areas bearing negative electrostatic charges;and, heating said surface to fuse said developer particles thereto andto remove said dimethyl polysiloxane therefrom.

References Cited in the file of this patent UNITED STATES PATENTSCarlson Oct. 6, 1942 Greig et a1. Feb. 21, 1956 Walkup Mar. 5, 1957Mayer Mar. 10, 1959 Mayer June 9, 1959 Mayer June 30, 1959 StraughanAug. 11, 1959 FOREIGN PATENTS France Nov. 9, 1955 Great Britain Aug. 22,1956 OTHER REFERENCES Dow Corning Silicone Notebook, Fluid Series No. 3,

17. IN A METHOD OF DEVELOPING AN ELECTROSTATIC IMAGE ON AN INSULATING SURFACE, THE IMPROVEMENT COMPRISING APPLYING TO SAID SURFACE A DEVELOPER COMPOSITION CONSISTING ESSENTIALLY OF FINE-LY ELECTROSCOPIC THERMOPLASTIC PARTICLES DISPERSED AS A PHASE IN A CARRIER LIQUID CONSISTING ESSENTIALLY OF A DIMETHYL POLYSILOXANE HAVING A VISCOSITY OF BETWEEN ABOUT 0.6 AND 3 CENTISTOKES, DEPOSITING SAID THERMOPLASTIC PARTICLES ON SAID SURFACE BY ELECTROSTATIC ATTRACTION THERETO, AND HEATING SAID SURFACE TO FUSE SAID PARTICLES THERETO AND REMOVE SAID DIMETHYL POLYSILOXANE THEREFROM. 